The present application applies to radiation scanners, such as computed tomography (CT) scanners. It finds particular application with the arrangement, or rather configuration, of anti-scatter collimators, including one- and two-dimensional types, within such scanners.
CT scanners typically comprise a radiation source and a detector array positioned on a diametrically opposing sides of a rotating gantry. During a scan of an object, the object is placed in an examination region of the scanner and the rotating gantry rotates about the object while radiation is emitted from a focal spot of the radiation source.
Radiation that impinges upon the object is attenuated as it traverses the object. Generally, highly dense objects attenuate more radiation than less dense objects. In this way, characteristics of the object, or rather internal aspects of the object, may be identified based upon the attenuation.
Radiation that traverses the object is detected by one or more pixels, or channels of the detector array and a signal is generated in response thereto. The signal is indicative of characteristics of the radiation that is detected by the pixel, and thus is indicative of the attenuation of the object in a particular projection. An image can be reconstructed from a set of projections, which represents density distribution within an object. In this way, an image may depict a high density object, such as a bone, surround by less dense tissue, for example.
In an ideal environment, the radiation that is detected by a pixel corresponds to attenuated radiation that strikes the pixel on a straight axis from the focal spot of the radiation source. This type of radiation is commonly referred to as primary radiation. Unfortunately, some of the radiation that impinges upon the object is scattered, and deviates from a straight path (e.g., due to inevitable interactions with an object). Scattered radiation that is detected by a pixel, commonly referred to as secondary radiation, increases noise and reduces the quality of an image produced based upon the detector signal. In diagnostic imaging, secondary radiation can account for as much as 90% or more of the total signal response that is generated by a pixel if no anti-scatter collimator is used.
In order to reduce the possibility of scattered radiation impacting a pixel of the detector array, anti-scatter collimators are commonly inserted between the examination region and the detector array. Anti-scatter collimators comprise anti-scatter plates configured to absorb scattered radiation and transmission channels configured to allow primary radiation to pass through the collimator and be detected by a pixel of the detector array. To promote capture of scattered radiation, the height (e.g., in a dimension extending from a detector to the radiation source) of the anti-scatter plates is generally larger that the width, or transverse dimension (e.g., in a dimension perpendicular to the height), of the transmission channels. This is commonly referred to as a high aspect ratio.
While the anti-scatter collimators have proven effective for capturing scattered radiation, anti-scatter plates impose “shadows” on the detector array. A pixel that is at least partially shadowed by an anti-scatter plate generates a signal which is reduced in strength relative to a signal from a non-shadowed pixel. A signal with a reduced strength can be corrected for if the shadow is substantially static. However, if the shadow is dynamic, the pixel may produce an unstable signal and cause artifacts to be produced in a resulting image.
A shadow may be dynamic for a plurality of reasons. For example, a dynamic shadow may be caused by focal spot motion due to thermal effects and/or from vibration caused during rotation of the radiation source. In another example, dynamic shadow is caused by bending of the anti-scatter plates. The long (e.g., 15 mm) and slender (e.g., 0.1 mm) design of the anti-scatter plates make them susceptible to bending during rotation. Further, the anti-scatter plates may be bent during the manufacturing process.
The effects of dynamic shadows may be reduced if the percentage change by respective pixels is uniform (e.g., a first shadow and a second shadow both increase by two percent). However, achieving a uniform percentage change has proven difficult for numerous reasons. For example, machine tolerances often cause the anti-scatter plates to not be aligned perfectly and/or cause the anti-scatter plates not to be the same width. Therefore, the spacing between anti-scatter plates may not be uniform and/or an anti-scatter plate may be positioned incorrectly relative to a pixel. Additionally, the anti-scatter plates may not bend uniformly so the percentage change may not be uniform. Therefore, it is difficult to reduce the effects of a dynamic shadow.